Hearing aids are normally divided into three categories: behind the ear (BTE) aids, in the ear (ITE) aids, or completely in the canal (CIC) aids. From the purely technical point of view, such hearing aids can be configured as so-called analog aids, as digital aids, or as aids with a combination of analog and digital techniques. The present invention can be used in connection with almost any form of hearing aid.
When a user switches on a hearing aid before it is positioned correctly at or in the ear, a high howling tone frequently arises because acoustic coupling between the sound transducer, i.e., a miniature loudspeaker, and the sound receiver, i.e., a microphone, makes the aid oscillate. This howling tone stops when the hearing aid is positioned correctly on or in the user's ear. BTE and ITE hearing aids usually have a mechanical on/off arrangement, e.g. a pushbutton, so that the user can wait to switch the aid on until it is correctly placed, thereby avoiding the oscillation. However, in order to ascertain whether the aid is functional, and whether there is a usable battery in the hearing aid, many users switch the hearing aid on before it is positioned at or in the ear. CIC hearing aids are so small that they rarely have a specific on/off arrangement. Such aids are switched on when a battery is inserted in the battery compartment and the battery cover is subsequently closed. When the battery cover is closed, the hearing aid is immediately switched on. That is, these types of aids are switched on before they are placed in the ear canal. The user can, thus, not avoid the howling tone until the aid is in place, which can be very troublesome. The howling tone can be extremely disturbing, especially for users of CIC hearing aids, since these are often used by people with a slight or a moderate hearing loss.
To facilitate the understanding of the invention initially the following definitions are given: “Start up period” is a time period that consists of the sum of two parts, namely, an “inherent delay period” and a “comfort delay period”.
The “inherent delay period” depends on the electronic design of the hearing aid, and the inherent delay is usually minimized during the design phase. If the hearing aid involved is of the analog type, a charging of supply and coupling capacitors, and possibly other components, must take place before the aid is completely functional. If the aid involved is of the digital type, programs or parts thereof must be read from non-volatile storage areas and transferred to volatile memory areas, such as program RAM and/or data RAM. The inherent delay period for a hearing aid is usually, at the most, around 0.5 seconds, and up to approximately 1 second for digital aids, during which program instructions and program data must be read. Efforts are being made, however, to make digital hearing aids functional within the range of 0.5–1 seconds or less.
The “comfort delay period” is an additional time delay period wherein the acoustic signal from the microphone is, partly or wholly, prevented from reaching the sound transducer.